# Pressure in a bomb calorimeter at the moment of combustion

• Seibtsantos
In summary, the conversation involved calculating the number of moles of glucose, checking the combustion reaction, and considering the number of moles at the end of the reaction. The final step was using the calculated number of moles in the equation pV = nRT to find the pressure (155.8 atm) at a temperature of 1200 °C. However, this answer may need to be rounded to 156 atm.
Seibtsantos
Homework Statement
The calorific value of food can be determined in a bomb calorimeter, which consists of a hermetically sealed stainless steel container, in which the sample is burned in the presence of pure oxygen and the resulting heat measured by the temperature variation of a water bath surrounding the container. Due to the thickness and mass of the container, the water bath undergoes a few degrees variation, however at the time of combustion the internal temperature can reach 1200°C.
In determining the heat of combustion of a glucose sample (C6H12O6), 1.8 g of this substance was placed in a calorimetric pump and the container (1 L capacity) was pressurized to 30 atm with pure oxygen (1.23 moles ) and the ignition perpetrated.
Consider: The adiabatic system, a complete combustion at 1200 °C and that the gases generated are ideal gases. Data: R = 0.082 atm.L.K-1.mol-1; Molar Mass (C6H12O6) = 180 g/mol.
The internal pressure of the container at the exact moment of the sample combustion is:
a) 124 atm.
b) 145 atm.
►c) 152 atm.
d) 163 atm.
e) 173 atm.
Relevant Equations
Ideal gas law
First, I calculated the number of moles of glucose.

n = m / M
n = 1.8 / 180
n = 0.01 moles of glucose

So I checked the combustion reaction.

1 C6H12O6 + 6 O2 -> 6 CO2 + 6 H20
1 + 6 -> 6 + 6
0.01 + 0.06 -> 0.06 + 0.06

I considered the number of moles at the end of the reaction.
I subtracted the number of moles from the initial oxygen by the oxygen consumed.

1.23 - 0.06 = 1.17 moles

I calculated the total number of moles.

0.06 + 0.06 + 1.17 = 1.29 moles

So I used it in the equation.

pV = nRT
p = (nRT) / V
p = (1.29 * 0.082 * 1473) / 1 (considering 1200 ° C = 1473K)
p = 155.8 atm

I didn't get to the result and I don't know what is missing

Thank you!

Your working and answer look OK to me. I'd round to 156 atm though, as you can't justify 4 significant figures. 'Official' answers are wrong occasionally. I guess that's what's happened here.

## 1. What is a bomb calorimeter?

A bomb calorimeter is a device used to measure the heat and energy released during a combustion reaction. It consists of a sealed container, or bomb, in which the reaction takes place, surrounded by water and a thermometer to measure the temperature change.

## 2. How does a bomb calorimeter measure pressure?

A bomb calorimeter measures pressure indirectly by measuring the temperature change caused by the combustion reaction. As the reaction takes place, the gases produced will increase the pressure inside the bomb, causing the temperature of the surrounding water to rise. This change in temperature can then be used to calculate the pressure inside the bomb using the ideal gas law.

## 3. Why is pressure important in a bomb calorimeter?

Pressure is important in a bomb calorimeter because it affects the accuracy of the measurement. As the pressure inside the bomb increases, the temperature of the surrounding water will also increase, leading to a higher recorded heat release. Therefore, it is important to accurately measure and account for the pressure inside the bomb to obtain accurate results.

## 4. How does pressure affect the results of a bomb calorimeter experiment?

Pressure affects the results of a bomb calorimeter experiment by influencing the temperature change recorded during the combustion reaction. As mentioned before, a higher pressure inside the bomb will result in a higher temperature change, leading to an overestimation of the heat released. Therefore, it is important to control and measure the pressure accurately to obtain reliable results.

## 5. What factors can affect the pressure inside a bomb calorimeter?

There are several factors that can affect the pressure inside a bomb calorimeter, including the amount and type of fuel used, the amount of oxygen present, and the temperature of the surrounding water. Additionally, any leaks or malfunctions in the bomb or its components can also affect the pressure measurement. It is important to carefully control and monitor these factors to obtain accurate results.

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